PO30 Presentation Time: 10:30 AM

Purpose

The utility of shielded cylinder intracavitary brachytherapy applicators in clinical practice has not been reported. We evaluated patients treated using this applicator to report its use in clinical practice and toxicity following treatment.

Materials and Methods

This single institution retrospective study included all patients who received high-dose rate (HDR) intracavitary brachytherapy using a shielded cylinder applicator (#GM11003380, Varian Systems, Palo Alto, CA) as a component of treatment between July 2013-November 2021. Treatment parameters, dosimetry; tumor and treatment characteristics were abstracted from the medical record. Patient toxicities were collected prospectively through an institutional radiation toxicity database.

Results

A total of 48 patients received 188 HDR intracavitary brachytherapy treatments, with 157 using a shielded cylinder. The majority (90%) also received external beam radiation (EBRT) to a median dose of 4500cGy in 25 fractions (interquartile range, IQR, 4500-5625cGy). Common fractionations (fx) for brachytherapy included 2000cGy/4fx with EBRT for patients with recurrent uterine cancer, 2000-2800cGy/4fx with EBRT or 3000cGy/5fx without EBRT for patients with primary vaginal or vulvar cancer, and 500cGy/1fx with EBRT for rectal cancer patients. The median total EBRT + brachytherapy EQD2 was 6930cGy (IQR 6930-7900cGy). In the treatments delivered with a shielded cylinder, shielding was used to reduce dose to rectum (51%), uninvolved vagina (34%), and bladder/urethra (8%). The most common shielding pattern was 180° (78%) and 90° (20%), with a minority of treatments using 270° shielding (2%). In cases where shielding was used to reduce rectal dose, the D2cc[Gy] rectal dose in delivered plans was 1.18Gy (range 0.53 - 3.92Gy). In patients who had both unshielded and shielded treatment plans, the D2cc[Gy] rectal dose was reduced from 4.07Gy to 1.69Gy, respectively. The median total EQD2 rectal dose in patients who received EBRT in 25 fractions in addition to brachytherapy was 38.9Gy (range 14.8 - 47.8Gy). Acute grade 2-3 toxicities included dermatitis (9), cystitis (4), proctitis (3), fatigue (2), nausea (2), pain (2), and anemia (1). In patients who received brachytherapy alone there were no grade 2+ toxicities reported. Late grade 2-3 toxicities reported 6+ months after radiation therapy included vaginal stricture (4), vaginal dryness (1), fatigue (1), lower GI hemorrhage (1), lymphedema (1), urinary frequency (1), and urinary incontinence (1). One patient had a grade 3 vaginal fistula noted at 6-months. There was one grade 2 sacral insufficiency fracture noted at 4-months. There were no acute or late grade 4-5 toxicities. Median follow-up was 35 months (IQR 11-55 months). Fifteen patients died, 6 from cancer progression and 9 from non-cancer causes. Fifteen patients had cancer progression at a median of 28 months from completion of brachytherapy. Five patients had local-only failures, 9 patients had distant-only failures, and 1 patient had pelvic nodal failure. There was no difference in total EQD2 delivered to tumor between those patients who had local failure versus local control, with a median EQD2 of 69.3Gy and 69.5Gy, respectively. The location of local failure was in the unshielded area in three patients and involved the shielded area in two patients.

Conclusions

Shielded cylinder applicators can be utilized successfully in the treatment of well selected patients with primary and recurrent gynecologic cancers and rectal cancers, enabling dose reduction to non-target structures with low treatment-related toxicity.